I have done ALL the work for this troublesome achievement. All you have to do is put the gnome inside and close the door. Notice that the door is noclipped so when you have put the gnome in you can still pull it back out.

Pros: Really useful, I could have done it mahself but, eh...
Improvements: Maybe make it a save where its right before that last strider is killed. Or when the rocket is going through the air. But this is still ok :P